This application relates to an improved erase system for multi-cell gas panel displays which provides a selective burst of bipolar signals to the particular cell in which erasing is desired.
Gas panels of the type to which this invention relates have two glass plates that are spaced apart by a seal to contain an ionizable medium. A set of horizontally extending insulated conductors are located on one glass plate and a set of vertically extending insulated conductors are located on the other plate. When a suitable voltage is applied between one horizontal conductor and one vertical conductor, ionization occurs at the crossover point of the two conductors and light is emitted. The crossover points are called cells, and a display pattern is formed by ionizing selected cells. The operation of initially ionizing a cell is called writing. Once a cell is written it is sustained by a continuously alternation potential called the sustainer. The operation of removing the wall charges from a previously written cell is called erasing. A cell is erased by applying a suitable voltage waveform to produce a controlled ionization so that the wall charge is reduced to or near to zero in the cell to discharge the cell. One object of this invention is to provide improved waveforms for erase operations.
As a result of the ionization that occurs during writing, positive and negative charges accumulate on opposite insulating walls of the cell. The voltage of this charge opposes the voltage applied between the vertical conductor and the horizontal conductor so that the sum of these voltages quickly falls below the voltage required for ionization and light is emitted from the cell for only a brief instant. The write voltage waveform is maintained for a sufficient interval after the light is extinguished for a substantial charge to be stored on the cell walls. After the write operation, periodic light output of the cell is sustained by an alternating polarity voltage that is called a sustain voltage. The sustain pulse following the write operation is opposite in polarity to the write pulse and thus is of the same polarity as the charge that was stored on the cell walls by the preceding write operation. Since the cell ionizes at a voltage that is the sum of the applied voltage and the voltage that is the sum of the applied voltage and the voltage of the stored charge, a previously written cell ionizes at an applied sustain voltage that is less than the write voltage. The sustain voltage is applied simultaneously to all cells and the previously written cells ionize and accumulate charge for the next sustain alternation but the previously erased cells with zero wall charge remain un-ionized.
A possible explanation for ionization in a gas panel will be helpful for understanding this invention. Independently of any voltage on the conductors of a cell, the cell medium ordinarily contains some free electrons and positive ions, and pilot lights may be located around the edge of the panel to establish a suitable level of ionization. The electrons and positive ions recombine and new ions are formed at an equalibrium rate. When a voltage is applied across the conductors of a cell, an electric field is formed in which the ions are accellerated so that ions collide more frequently with neutral atoms and thereby produce additional ions. At relatively low voltage levels an equalibrium condition may be reached where there is a high level of ionization but ions are lost by recombination as fast as they are created by collisions between atoms and ions. However, at some higher voltage level, ions are created faster than they are lost and these ions in turn produce additional ionization so that an avalanche of free charges occurs. Thus, both of the height and the width of the cell voltage waveform are important in establishing whether avalanche ionization will occur. As has already been explained, avalanche ionization is required for write, erase, and sustain operations.
It has been found that in attempting to do a selective erase operation, particularly in the case of erasing a cell which is surrounded by a number of lit cells to remain illuminated, the erase operation may be ineffective. It is believed that there are at present three basic techniques of erasing gas panel cells. One of these is the so-called narrow high amplitude erase which has the advantages that it erases better at Vs max. (sustain voltage maximum), it is insensitive to amplitude charges, it is selective and it is fast. It has the disadvantages that the high amplitude requires high voltage circuits for deselect and it is sensitive to width tolerances.
Another technique is the wide low amplitude erase which has the advantages that it is insensitive to width tolerances, low voltages may be used for deselect since it is a low amplitude erase, it is selective, and it is fast. It has the disadvantages that erasing at Vs max. may require critical amplitude adjustment and it is sensitive to amplitude tolerances.
Still another erase technique applies a burst of narrow sustain pulses which has the advantage that it is insensitive to width tolerances and erases at Vs max. very well. It has the disadvantages that it can not be applied selectively to only a desired cell or cells since it is on the sustain voltage and it is slow.
The subject improved erase technique has all of the advantages of the prior known erase techniques and none of the disadvantages. It is accordingly an object of this invention to provide a gas panel erase pulse technique that erases very well at Vs max., is insensitive to amplitude charges, is insensitive to width charges, is low amplitude and can be made selective with low voltage circuits for deselects, and is fast.